I make my Knob closed hatch covers with a wood stiffener like ross sudgests. I use ash or mahogany.

You may be able to trace the curve that the covers should be off the inside of the hatch opening, otherwise strap the hatches down and make a cardboard cutout that fits the outside of the hatch to use as template for makeing the stiffeners. if you use the outside of the hatch as a guide don't forget to offset the template by the thickness of the hatch, ie make the stiffener smaller. i use 2 stiffeners on most hatches, 3 on large hatches.

Be sure you do not put them on the hatchcover where they will interfere with closing the hatch.